Could you just give the tower a behavior that has a periodic effect that searches the area close to it for worker units?
Or like the person above specified, you could do it via triggers. I bet you could specify the "Event" as being "Unit is Issued Order" with the Move ability, and then have conditions to make sure the Unit Type of Triggering Unit is a worker and the Unit Type of "Target Unit for the Order" is a tower. For Actions, I'd have the trigger issue the Triggering Unit to use an ability (not on the command card) on the tower (Target Unit for Order) called "Tower capture" or something like that with a really short range. And then either another trigger can detect when that ability's effect occurs, or you could make the ability itself accomplish the change in ownership.
Hopefully there's a decent idea in there-good luck!
Here is a map with my idea implemented-it works, but you'll have to make it look good!
Could you just give the tower a behavior that has a periodic effect that searches the area close to it for worker units?
Or like the person above specified, you could do it via triggers. I bet you could specify the "Event" as being "Unit is Issued Order" with the Move ability, and then have conditions to make sure the Unit Type of Triggering Unit is a worker and the Unit Type of "Target Unit for the Order" is a tower. For Actions, I'd have the trigger issue the Triggering Unit to use an ability (not on the command card) on the tower (Target Unit for Order) called "Tower capture" or something like that with a really short range. And then either another trigger can detect when that ability's effect occurs, or you could make the ability itself accomplish the change in ownership.
Hopefully there's a decent idea in there-good luck!